Cleanup: test_bitfield: nr_bits should be unsigned

Cleanup: test_bitfield: nr_bits should be unsigned

Should not have any effect on the code behavior, but removes
useless type conversions between unsigned and signed types.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I39096ab89936875009a68d3ab8a4546c7f1eb8ae
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1375
CI-Build: Michael Jeanson <mjeanson@efficios.com>
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>